- The presence of unreacted, free silicon lowers the mechanical properties of reaction-bonded boron carbide. The fraction of free silicon can be reduced by increasing the green density of the initial boron carbide performs. The use of multimodal boron carbide mixtures allows attaining 75% green density. After reaction bonding with molten silicon, the composites consist of four phases, namely the original B 4 C particles, the B 12 (B, C, Si) 3 phase, product of the dissolution–precipitation process, β-SiC, and residual Si. The volume fraction of residual Si in the composites is in the 8–10% range. The infiltrated composites display elevated values of the mechanical properties with a high Weibull modulus.